In the
surface world, basically they were and still are a way of countering the ASW threat, evasive steering was/is used to "deny the enemy submarine a valid fire control solution".
There were different plans for different scenarios, taking into account different conditions, convoys fast or slow, just warships on their own, the overall speed required to make the destination etc.
A base course would be signalled with a time check, then an 'in force plan' would be signalled, all ships having a copy of the relevant publication, this plan would then tell the convoy at what time past the hour to steer a course 'off' the base course, for example if they were steering 270, the first zigzag course would be 15 degrees to port then say 20 minutes later it would order them to steer 23 degrees to starboard and so on, the idea being that they would 'zig zag' along the base course, some plans had course changes every 2 to 3 minutes others 30 minute interval for example.
I don't know if this is what is meant in your book, I'd imagine it'll be something similar so they can escape detection and attack.
http://www.history.navy.mil/library/online/warinst-7.htm